1. Field of the invention
The invention relates to a method of patterning metal on a dielectric substrate during fabrication of an electronic component, and more particularly to masklessly depositing a thin etch mask on a dielectric by direct-write prior to selectively etching channels in the dielectric and depositing conductive metal in the channels.
2. Description of Related Art
To form circuits, semiconductors and other electronic components need to be interconnected with each other. As semiconductors continue to improve in performance, cost, reliability and miniaturization, there is an intensive need in the electronics industry, especially for large scale computers, to package and interconnect these semiconductors without limiting system performance. One approach is mounting the components on high density multichip modules (electrical interconnect substrates). These modules normally contain buried electrical lines or channels which terminate at bonding pads on a mounting surface. By bonding electrical terminals on mounted components to bonding pads on the modules, multiple components can be electrically interconnected.
Applicant's recent cost modeling on fabrication of high density copper/polyimide modules reveals that, regardless of the fabrication process, the three costliest materials are the polyimide, substrate base, and photoresist chemicals. Polyimide cost can be reduced by replacing spin coating with less wasteful approaches, such as extrusion coating. Polyimide expenses can also be reduced by choosing polymers other than traditional polyimide. Photolithography tends to be the most expensive step. During conventional photolithography, resist material is deposited on a smooth upper surface of a layer, the resist layer is photoexposed through suitable artwork to define a pattern of areas where undeveloped resist (which may be exposed or unexposed) is washed away. An etch step is performed on the underlying layer, and the resist is stripped by wet chemicals. As such, photolithography not only requires expensive chemicals and aligners/steppers, but also is typically the most time consuming and labor intensive step. Furthermore, traditional photolithography optical aligners require very flat substrates due to the small depth of focus. This becomes an increasingly major drawback as larger substrates are used. Needless to say, any patterning technique without conventional photolithography is potentially valuable.
Other methods have formerly been developed in order to overcome the drawbacks of photolithography. The main thrust has been to develop several maskless (or re-usable mask) techniques to direct-write metal lines and features on substrates. Such techniques include liquid metal ion sources, liquid metal cluster sources, laser direct-write, chemical vapor deposition, ink jet printing, offset printing, palladium activated plating, and electron-beam enhanced deposition. These techniques have matured into relatively reliable processes. However, at present, none of these techniques have been widely accepted for manufacturing. Drawbacks arising from these techniques include low throughput, poor adhesion, high resistivity, high contact resistance, and poor resolution. Furthermore, the low deposition rates inherent in direct-write limit its usage to specific applications such as repair and fabrication of very thin lines (0.1 to 1 microns thick). But current multi chip modules typically require thick metal lines (on the order of 5 to 10 microns) and thus current state-of-the-art direct-write techniques can not form such lines in a practical manner.
The use of direct-write to form a mask for patterning metal lines has been described in U.S. Pat. No. 4,612,085 by Jelks et al. (hereinafter the '085). More particularly, the '085 describes a method of forming a molybdenum oxide plasma etch mask by selective pyrolytic photochemical decomposition on either a metal or dielectric surface. The process of the '085 is well suited for fabricating 0.5 to 1 micron thick features but unsuitable for fabricating 5 to 10 micron features for several reasons. First, the dry etch rate for most conductive metals of interest is so small as to nearly equal the dry etch rate for the mask. For instance, suppose 10 micron thick lines of copper (normally the conductive metal of choice) are desired. A mask approximately 10 microns thick becomes necessary to prevent erosion before the copper is patterned. But, as previously mentioned, direct-write of a 10 micron pattern (mask or metal line) is prohibitively time consuming. Furthermore, a wet etch is unsuitable for patterning metal lines with dimensions on the order of a few microns since isotropic undercutting would deform if not destroy the lines. Second, direct-write of a mask by the pyrolytic photochemical decomposition, as described in the '085, causes the mask features to grow laterally on each side as the thickness of the mask increases. This limits the aspect ratio of the mask openings (height/width) to 0.5. Third, pyrolytic photochemical decomposition suffers from inaccurate deposition placement due to scattered light. And fourth, deposition on the viewport through which the substrate is illuminated causes the process to be unreliable.
Therefore the related art does not teach how to pattern metal lines on a substrate without photolithography in a cost effective manner.